Since polyester films have an excellent thermal characteristic and a physical characteristic, they have been used in various application uses such as magnetic recording media, capacitors, flexible substrates, optical members, food packages, decorative use.
By the way, in magnetic recording media, particularly, magnetic recording media for use in data storage, requirement for characteristics to the base films has become severer along with increase in the capacity and the density of tapes. In magnetic recording media for data storage adopting a linear track system such as QIC, DLT and, further, high capacity super DLT, and LTO, the track pitch is made extremely narrow for attaining higher capacity of tapes. Therefore, they involve a problem of causing track deviation to generate errors in a case where dimensional changes occur in the direction of the tape width. The dimensional changes include those due to changes of temperature and humidity and due to aging shrinkage in the transverse direction which is caused upon repetitive running in high temperature and high humidity states under high tension. In a case where the dimensional change is large, it brings about track deviation to generate errors upon electromagnetic conversion. For the sake of convenience of explanation, the advancing direction when a film is formed continuously is referred to as a film-forming direction, a continuous film-forming direction, a longitudinal direction or an MD direction and, also, the direction within a plane perpendicular to the film-forming direction is sometimes referred to as a transverse direction or a width direction.
For solving such dimensional changes, JP-A No. 5-212787 discloses a biaxially oriented polyethylene-2,6-naphtalene dicarboxylate film in which the Young's modulus in the longitudinal direction (EM), the Young's modulus in the transverse direction (ET) and the ratio between both of the Young's modulus (ET/EM) are defined each within a predetermined range, and the shrinkage in the longitudinal direction, the temperature expansion coefficient in the longitudinal direction (αt), and the humidity expansion coefficient in the longitudinal direction (αh) are defined. Further, the pamphlet of WO No. 99/29488 discloses a biaxially oriented polyester film in which a thermal expansion coefficient αt (×10−6/° C.) in the transverse direction, a humidity expansion coefficient αh in the transverse direction (×10−6/% RH) and shrinkage P in the transverse direction to a load when the load is applied in the longitudinal direction (ppm/g) are defined each within a predetermined range. Furthermore, the pamphlet of WO No 00/76749 discloses a biaxially oriented polyester film in which the dimensional change in the transverse direction when left under weight in the longitudinal direction, thermal expansion coefficient αt (×10−6/° C.) in the transverse direction, the humidity expansion coefficient αh in the transverse direction (×10−6/% RH) and a shrinkage P in the transverse direction to a load when the load is applied in the longitudinal direction (ppm/g) are defined each within a predetermined range.
However, the methods proposed in the publications are attained by defining the stretching conditions and the subsequent heat setting treatment conditions within the predetermined ranges. For example, while the aging shrinkage in the transverse direction upon applying a load in the longitudinal direction can be improved by increasing the Young's modulus in the longitudinal direction of the base film, on the other hand, from a view point of the polymer characteristic and the film forming property as the Young's modulus in the longitudinal direction increases, the upper limit for the Young's modulus in the transverse direction decreases, which results in increase in the dimensional changes due to the temperature/humidity change, etc. and no drastic solution has not yet been attained.
Further, capacitors are manufactured by a method of stacking a thermoplastic resin film such as of polyethylene terephthalate or polypropylene and a thin metal film such as an aluminum foil and winding or laminating them. In recent years, along with the demand for size-reduction of electric or electronic circuits, size-reduction and mounting has been progressing also for film capacitors and further heat resistance has been required in addition to an electric characteristic. Further, in the application use for automobiles, the range of use is extended not only to the use in a driver's cab but also to the inside an engine room, and film capacitors suitable to dimensional stability at further higher temperature and higher humidity have been required in addition to the electric characteristic.
In view of the above, with an aim of solving the heat resistance of films for use in capacitors, JP-A No. 2000-173855 discloses a method of using a polyethylene-2,6-naphthalate film and, with an aim of improving the electric characteristic thereof, a method of controlling the state of crystallization, intrinsic viscosity, etc. has been proposed. However, the method has a limit in the further improvement of the electric characteristic since this is a polar polymer.
On the other hand, as a thermoplastic resin with an excellent electric characteristic, syndiotactic polystyrene polymers have been known. However, since the syndiotactic polystyrene polymers are more difficult to be formed into a film compared with polyester resins and since the obtained film tends to be torn as well, improvement for the handleability during manufacture of capacitors has been demanded.
By the way, the pamphlet of WO No. 97/32223 proposes films containing syndiotactic polystyrene and polyethylene-2,6-naphthalate. However, such films are optical materials for controlling the optical characteristic such as reflectance or transmittance and they are substantially monoaxially oriented films.
Further, JP-A No. 08-176329 proposes a void-containing polyester film in which a syndiotactic polystyrene is blended as a void former with a polyester resin and it discloses that the less deformability of the syndiotactic polystyrene at a stretching temperature gives an effect on the development of voids. However, as the thickness of the film decreases, since the effects of voids on various kinds of characteristics increase, there may be a possibility in the application use requiring thin film thickness, that various kinds of characteristics necessary for the application uses, for example, the mechanical characteristic such as Young's modulus and the withstand voltage characteristic are deteriorated.
Further, as a film formed by laminating a syndiotactic polystyrene and a polyester, JP-A No. 8-48008 describes a laminate film with the ratio of the syndiotactic polystyrene layer of 70% or more.
Further, JP-A No. 2000-326467 proposes a multi-layered laminate film in which a layer comprising polyethyelen-2,6-naphthalate and a layer comprising a syndiotactic polystyrene are alternately laminated by 11 layers or more. However, such a film intends to selectively reflect a light at a predetermined wavelength by optical interference due to the difference of refractive index between layers.